The invention disclosed herein relates to metal refining and particularly to a lance for injecting gas in a vessel containing molten metal to effect agitation and stirring of the metal.
In steel making, by way of example, it is customary to tap the refining furnace and let the melt flow into a refractory lined holding vessel for alloying and desulphurizing and performing other steps that are needed for the product to meet specifications. The melt is tapped when it is at a higher temperature than is optimum for ultimate utilization such as casting it into ingots or furnishing it to the tundish of a continuous casting machine. While undergoing alloying and other processing, the melt cools at a generally predictable rate and it has a tendency to develop strata having different chemical compositions and temperature gradients. Hence, stirring is required to make the melt as uniform as possible in all respects before it is poured from the holding vessel.
Stirring is usually effected by injecting a gas into the melt at a substantial distance below its surface. One prior method involves setting a porous plug of ceramic material in the refractory lined bottom of the holding vessel and forcing pressurized gas such as nitrogen or argon from the outside of the vessel through the plug. The resulting bubbles rise through the melt and induce circulation. The problem is that the action of the melt on the plug and the high temperature cause the pores at the surface of the plug where it interfaces with the metal to fuse and clog after use. Moreoever, when the melt is finally poured from the holding vessel some slag remains in the vessel and it bonds onto the plug and seals its pores. Consequently, after each use of the vessel a gas burning torch is used to burn away any accumulations on the plug and thereby expose a new clean porous surface. Porous plugs also erode after not many usages of the vessel to the point where replacement of the plug and rebuilding of the refractory bottom lining has to be undertaken.
Another prior scheme for stirring with gas used a ceramic plug set in the bottom refractory liner of the holding vessel but the plug was not porous. Rather, the plug was usually in the form of a truncated cone set in a complementarily shaped insert in the liner with spacers such that there was a small gap or free space around the surface of the cone. Thus, instead of relying on porosity of gas conduction, axial flow along the gap was used. While the integrity of the plug was maintained, gas evolved in a circular pattern from the end of the cone and entered the bottom of the melt for an upward ascent. However, this scheme turned out to have just about the same problems as the porous plug. As it eroded away, it imperiled the integrity of the vessel bottom so it and the bottom had to be replaced quite frequently. Residual slag froze over the circular gap exits and plugged the gap so the plug had to be burned clear between successive uses of the vessel as was formerly the case.
Before use of plugs in the bottom of a vessel became commonplace, lances were used to inject stirring gas into holding vessels. The first lance designs were essentially a pipe surrounded concentrically by a refractory material sheath. The open end of the pipe was positioned six inches, more or less, from the bottom of the vessel and the pressurized gas was forced out in opposition to the hydrostatic head of the molten metal. With this kind of lance the gas emitted from the lower end of the pipe flowed up through the melt in the form of large bubbles which did not agitate nor stir the melt effectively. Furthermore, when the large bubbles erupted from the molten metal surface they spewed and splashed molten metal which is undesirable.
Probably the most widely used gas stirring device in current use combines the lance and porous plug concept. A porous ceramic plug which has been fired and is dimensionally stable is encased in a metal cylinder which is connected to a long gas feed pipe. The pipe and cylinder are encased in a refractory material sheath to form a lance that is immersible throughout most of its length in molten metal. Pressurized gas fed down the pipe permeates the porous plug such that gas is emitted from the end of the plug in fine bubbles. Stirring effectiveness has been considered acceptable for the lack of anything better until the invention disclosed herein was made. However, the currently used lances still suffer plugging of the pores in the ceramic plug. Some clean-up work has to be done on them after each use if they are to be reused. Material has to be chipped away in order to restore porosity at the interface of the plug and the metal.